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duced to little more than the thickness of half-a-crown. Little 

 margaceous nodules, the mineral bezoars of some, composed of 

 various lamina? of soft earth, of different colours, may be frequently 

 found in the gravel-pits in England, and in large heaps in several 

 parts of Italy ; appearing to differ from silicious pebbles, only in 

 not having been impregnated with silicious matter. That such an 

 aggregation of particles of clay and sand, as may form a fit sub- 

 stratum of pebbles, may even now take place, and that these bodies 

 may attain the forms of pebbles by attrition and the action of water, 

 is very probable. In what manner that process, on which their 

 induration depends, is accomplished, is not, however, easy to ascer- 

 tain. 



The opinion of Mons. Reaumur, as to the manner in which this 

 part of the operation, in the formation of pebbles, is accomplished, 

 appears to be not only perfectly consonant with the appearances 

 which pebbles yield ; but also with those which are yielded by the 

 several silicious petrifactions, which are more particularly the ob- 

 ject of our inquiry *. He remarks, that, by a coarse operation, emery 

 is reduced to powder, and suspended in water, for several days ; 

 but, he observes, Nature may go much further ; for the particles 

 which water detaches from hard stones, by simple attrition, are of 

 an almost inconceivable degree of fineness. Water, thus impreg- 

 nated, he supposes, contributes to the formation of pebbles, by 

 petrifying of stone, as it were a second time : stones, already formed 

 but remaining of a spongy nature, acquiring a flinty hardness by 

 an impregnation with this crystalline fluid. Thus impregnated, 

 masses of different clays, chalks, marles, and boles, become flinty 

 stones ; and thus, he remarks, this crystalline fluid, filtering through 

 a porous lime-stone, will deposit its very minute particles, in all 

 the interstices, existing between the particles of the stone, which 



* Memoires de 1* Academic des Sciences, &c. 1721, p. 241. 

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